The bifunctional nickel-containing enzyme, carbon monoxide dehydrogenase/acetyl-CoA synthase (CODH/ACS), catalyzes the reduction of carbon dioxide (CO2) to carbon monoxide (CO) and the assembly of acetyl-CoA from CO in acetogenic organisms. The latter reaction is a rare example of a biological organometallic reaction sequence in which the intermediacy of Ni-(alkyl) and Ni-(acyl) species are proposed and is important to autotropic carbon-fixation processes in the biosphere. Recent crystallographic structure determinations of a CODH/ACS enzyme have revealed - what appears to be - a four-coordinate, tetrahedral Ni atom in the ACS active site. However, tetrahedral Ni-(alkyl) and Ni-(acyl) complexes have no precedent in coordination chemistry and the suggestion that the enzyme utilizes such centers for acetyl-CoA assembly remains controversial. The goal of this proposal is to address the role of tetrahedral Ni centers in the reaction sequences of ACS catalysis through the use of model complexes supported by a "linked" tris(pyrazolyl)borate ligand system. The "linked" tris(pyrazolyl)borate ligands proposed here possess the attributes necessary to mimic the tetrahedral geometry of the Ni centers in the ACS active site and to provide isolable organometallic Ni species relevant to acetyl-CoA assembly. Reactivity studies of the organometallic Ni model complexes with relevant biological substrates are proposed which will delineate potential reactivity patterns available to the tetrahedral Ni center in the ACS active site and catalytic function.